Fig. 1.

Catalytic cycle of mARC enzymes. Reducing equivalents supplied by NADH are passed to cytochrome b₅ reductase, then to cytochrome b₅, before being relayed to mARC to prime the Mo^IV active site for substrate reduction. It is assumed that N-hydroxylated substrates are reduced by cleavage of the N–O bond, in analogy to the described mechanism of nitrate reduction by nitrate reductase (17). This is accomplished by protonation of the hydroxyl group and subsequent leaving of one water molecule (18).